The present invention relates to a CDMA (Code Division Multiple Access) transmission/reception system and, more particularly, to a CDMA transmission/reception system generally used for a forward link (communication from a base station to a mobile state) in a mobile communication system.
A code division multiple access (CDMA) scheme based on a direct spread technique has received a great deal of attention as a multiple access scheme in a future mobile communication system. This is because the CDMA scheme has the potential for a great increase in subscriber capacity. In a system using the CDMA scheme, each user multiplies information signal by a unique spread code to spread the signal into a signal in a wide frequency band, and transmits the resultant signal to a transmission channel. On the reception side, the code multiple signal undergoes a de-spread process, and the desired user signal is detected afterward.
FIGS. 11A and 11B show a conventional CDMA transmission/reception system. FIG. 11A shows a transmitter. FIG. 11B shows a receiver. In this transmission/reception system, the transmitter multiplexes user signals to receivers, i.e., mobile stations, altogether. In the transmitter, the user signals and pilot signals are multiplexed to allow the receiver to easily estimate the reception timings of the respective user signals and transmission channel parameters.
Referring to FIG. 11A, in the transmitter, user signal spreading circuits 501-1 to 501-K (K is an integer not less than 1; the same applies to the following description) spread the respective user signals with unique codes assigned to the respective users. A pilot signal spreading circuit 502 spreads a pilot signal with a unique code. A signal combiner 503 multiplexes the spread user signals supplied from the user signal spreading circuits 501-1 to 501-K and the pilot signal from the pilot signal spreading circuit 502. A transmission antenna 504 transmits an output from the combiner 503.
Referring to FIG. 11B, in the receiver, a pilot correlator 601 detects the pilot signal from the received signal transmitted from the transmission antenna 504 and received by a reception antenna (not shown), and extracts the reception timings of the user signals and transmission channel parameters. Similarly, a user signal correlator 602 detects a desired user signal from the received signal. A coherent detector 603 demodulates a user signal by using the transmission channel parameters extracted from the pilot signal. Note that illustrations of an RF (radio frequency) transmitter and a receiver are omitted from FIGS. 11A and 11B.
In a mobile communication environment, fading caused in a transmission channel generally becomes a factor for a deterioration in reception quality. As a means for improving the reception quality in a fading environment, for example, an antenna diversity scheme is effective, which is disclosed in Japanese Patent Application No. 6-189293 (Title of the Invention: “Code Division Multiple Receiver”) which is a pending application filed by the present applicant. This scheme is a selection scheme in which a plurality of antennas are arranged in a receiver to have independent spatial correlation characteristics, and a signal having good quality is selected from signals received by the antennas, or a combining scheme in which signals are appropriately weighted and combined. With the use of such a scheme, the reception characteristics in a fading environment can be improved. However, it is difficult to apply the scheme to a mobile station, because a complicated apparatus is required.
In the CDMA transmission/reception system shown in FIGS. 11A and 11B, in detecting a desired signal, interference occurs owing to the correlation between the desired user code and other users codes. Various interference cancellation schemes have been proposed (e.g., Japanese Patent Laid-Open No. 7-030519) to cancel such interference in a receiver and attain high reception quality. However, it is not easy to apply an interference canceler to a mobile station, considering a shortage of information associated with other users and the apparatus size. Therefore, there is a demand for a means for obtaining an interference reduction effect without applying an interference canceler to a mobile station.
In order to meet this demand, there has recently been proposed a scheme of obtaining a diversity effect and an interference reduction effect without using a plurality of antennas and an interference canceler in a mobile station. This scheme is disclosed in Miya, Hayashi, Kato, and Homma, “A Base-Station-Based Diversity Scheme for CDMA/TDD Systems”, TECHNICAL REPORT OF IEICE. RCS94-73, September 1994. The scheme is applied to a TDD (Time Division Duplex) scheme, i.e., a scheme in which communication is performed by time division of the same radio frequency in transmission and reception, and the same transmission channel is used for transmission and reception. In a base station, antenna diversity is performed in a reverse link, and forward link transmission is performed through an antenna exhibiting the most effective reception characteristics. With this operation, a diversity effect can be obtained in a mobile station without using a plurality of antennas.
In the CDMA scheme, with the user of such a transmission diversity scheme, an interference reduction effect can be expected for the following reason. A desired signal is always transmitted through an optimal transmission channel, whereas a signal of another users, i.e., interference, is not always transmitted through an optimal transmission channel for the desired user. A decrease in interference power can be attained unlike the case wherein all user signals are transmitted from a single antenna as shown in FIGS. 11A and 11B.
As has been described above, in the conventional CDMA transmission/reception system shown in FIGS. 11A and 11B, a complicated mobile station is required to improve the fading and interference resistance characteristics. Although there are some merits in the method of performing transmission diversity using the TDD scheme, a complicated system is required because synchronization in transmission and reception must be established between base stations.
In contrast to this TDD scheme, an FDD (Frequency Division Duplex) scheme allows a simple system. In this scheme, different radio frequencies are used in transmission and reception. A demand therefore arises for a means for obtaining the above effect in the FDD scheme.